Semiconductor wafer fabrication facilities typically include a number of wafer processing systems located in a clean room. The processing systems may include ion implanters, annealers, diffusion furnaces, sputter coating systems, etching systems and the like. Semiconductor wafers are transferred from system to system for processing in accordance with a predetermined schedule. Wafers have historically been transferred in open containers such as cassettes, either manually or using various transport systems.
A number of trends are apparent in the semiconductor wafer fabrication industry. Wafers are becoming larger, up to 300 millimeters in diameter, and device geometries are becoming smaller. A finished wafer may be worth as much as $250,000. Thus, extreme care is required in the handling of wafers to avoid even the slightest damage. Furthermore, as semiconductor device geometries become progressively smaller, the allowable particulate contamination specifications become more restrictive. In particular, particulate contamination specifications have been reduced by two orders of magnitude because of the reduction in device geometries. One of the steps taken to meet the particulate contamination specifications is to store and transport wafers in enclosed wafer containers known as front opening unified pods (FOUP's). Wafer pods typically store up to 25 wafers and have a door that is opened for access to the wafers.
Another trend is toward distributed stocking of the wafers in process. Thus, a quantity of wafers is stocked at each processing system to ensure that the processing system is not idle waiting for wafers to process. In particular, processing protocols may require stocking of a one hour backlog of wafers at each processing system. At each processing system, the wafers must be unloaded from the wafer pods and transferred into the processing system. Following completion of the processing, the wafers must be unloaded from the processing system and replaced in the wafer pods for transport to the next processing system or to a storage facility.
Encompassing both the larger wafer size and the distributed wafers in process, another trend is to minimize the process tool and wafer storage footprints due to the increasing cost of semiconductor manufacturing factories. Typically, semiconductor device manufacturers prohibit footprint increases due to the larger wafer size unless a proportional wafer processing throughput occurs.
Accordingly, it is desirable to provide an automated system for handling and storing wafer pods, and for transferring wafers to and between the wafer pods and the processing system, while meeting or exceeding particulate contamination specifications.